1. Field of the Invention
This invention relates to a process for the production of low-viscosity, storable amphoteric surfactants in which imidazolines are quaternized or carboxymethylated, hydrolyzed and stored in narrow pH ranges.
2. Statement of Related Art
Amphoteric surfactants, more particularly of the imidazolinium betaine type, have favorable foaming and cleaning properties and are acquiring increasing significance as co-surfactants in manual dishwashing detergents and cosmetic cleansing and personal care products [Seifen-Fette-Ole-Wachse, 108, 373 (1982)]. Surfactants such as these are normally obtained by alkylation of imidazolines with sodium chloroacetate and subsequent or simultaneous hydrolysis of the imidazoline ring. A particular problem of the production process is to obtain amphoteric imidazolinium surfactants which have low viscosities, even in the event of prolonged storage, and which are therefore easy to pump and dose.
There has been no shortage of attempts in the past to produce amphoteric surfactants of the imidazoline betaine type. Two different routes have been adopted for this purpose, namely:
1. Single-stage process: the reaction with sodium chloroacetate takes place after the hydrolysis of the imidazoline ring [cf. GB-A 850,514, GB 930,296, U.S. Pat. No. 2,773,168, EP-B 0 040 346 (Henkel), DE 36 39 752 (Kao)]. According to the teachings of these documents, however, highly viscous products with a target solids content of around 50% by weight are always obtained. PA0 2. Two-stage process: in the first step, the imidazoline is reacted to the betaine which is then hydrolyzed, i.e. ring-opened, in the presence of bases before reacting off with more sodium chloroacetate [cf. DE 20 63 424 (Rewo), U.S. Pat. No. 4,269,730 (Stepan), EP-B-0 001 006 (Albright & Wilson), DE 40 38 983 (Henkel)]. Although products produced in this way are of low viscosity, even in highly concentrated form, immediately after their production, they undergo a rapid increase in viscosity in the event of prolonged storage, ultimately becoming jelly-like substances which are no longer able to flow. PA0 a) 1-hydroxyethyl-2-alkyl-2-imidazolines corresponding to formula (I): ##STR4## in which R.sup.1 is an alkyl radical containing 5 to 21 carbon atoms, are quaternized or carboxymethylated with halogenated carboxylic acid salts and, at the same time, hydrolyzed with aqueous bases at a pH value in the range from 7.5 to 9 and PA0 b) the end reaction products are adjusted to a pH value of 5 to 7.
It is known from an extensive article by S. Takano and K. Tsuji in J. Am. Oil. Chem. Soc. 60, 1807 (1983) that the highly viscous products have a high content of open-chain monocarboxylated compounds corresponding to formula (IIa): ##STR2## By contrast, low-viscosity products mainly contain a dicarboxylated product corresponding to formula (IIb): ##STR3##
Accordingly, the problem addressed by the present invention was to provide an improved process for the production of low-viscosity, storable amphoteric surfactants. More particularly, the invention sought to provide amphoteric surfactants with a high content of compounds corresponding to formula (IIb).